I understand the (very successful) www.gov.uk site was largely done by direct government employees, working for the various departments the website covers.

On the other hand, a number of huge, subcontracted efforts (NHS system, tax IT consolidation) seem to be going much worse.

I don't think this has to do with the skill of the people involved. Rather, I think when the lack of flexibility in huge government contracts meets the demand of a new government to change everything, the project fails.

Yes, www.gov.uk is the product of the new Digital Government Service which I linked above. They haven't been around for very long yet and they are currently only responsible for the government web-services but I hope that their success will eventually pave the way for in-housing much bigger project likes NHS systems.